A terminal apparatus must be connected in a communication network or a switchboard in order to provide a variety of services relating to data, voices, etc. in ISDN. Functionalities should be defined in the process of terminal-switchboard connection. To this end, CCITT proposed connection functional structure between the subscription area and network where U-interface provides interface connection between existent subscribers and network, and S-interface provides that between terminals and network.
Conventional network terminals adopted a microprocessor for activation/non-activation, maintenance and repair and indication of the status of the system components. It should be pointed out that a separate power supply network had to be provided for energizing the network interface apparatus. Such conventional network terminal apparatus had the drawbacks that the use of a microprocessor make the system design complicated and costly and that stoppage of a regular power supply prevented the system from providing the services.